The increasing interaction between data communication equipment having varying communication characteristics has led to the adoption of standard protocols, such as the CCITT X.25, to facilitate compatible communications. Typically, these standard protocols are divided into a hierarchical set of levels or layers. The layering aspect of a protocol implementation separates communication functions into distinct protocol levels to enable communications between equivalent levels of equipment at each end of a facility.
One level of the X.25 protocol, called the high level data link control HDLC provides data link control which involves functions such as link initialization, the detection and recovery from errors in the transmission of numbered blocks of digital data, and data flow control to ensure that blocks of data are not being sent by one terminal faster than it can be received by the other terminal. The HDLC line link protocol specifies a repertoire of commands and responses to provide these data link control functions. Because of the variety of commands/responses and the resulting number of states in which the connected communication equipment can operate, the equipment requires sophisticated controllers to control link level communications.
In certain telephone applications, there is a need for line powered communication equipment or terminals which do not require the sophisticated communications capability provided by the standard HDLC protocol but which, nevertheless, require that communications be compatible with the standard HDLC protocol.